In my previous post I showed how to enable the Raspberry Pi 4 on Ubuntu Server to read out DHT11/DHT22 sensor data. But this is a very hacky solution. Moreover is the Raspberry Pi cluttered with some stuff, that we may need to configure differently for another setup or remove as a whole. It’s a nice coincidence that I’m currently reading Docker in Action, Second Edition (Manning, 2019), so I wanted to build a docker image to read out the data. There are some images available for this already, but I still wanted to build my own light-weight image.

Enough foreword, let’s jump right in.

Prerequesites

• Raspberry Pi 4 with Docker installed (curl https://get.docker.com/ | sh)
• DHT11/22 connected with your RapsberryPi (find any guide online for wiring)

Build Base Image

Interactively in a Container

This section is to interactively build the base docker image. If you want to get a Dockerfile right away, see the next section.

Start Container

As base we use python:3.7-alpine, as it is a comparatively very small image with only 41 MB size. We start the alpine shell /bin/ash and to attach us to the container we use the -it parameter. The --name dht parameter is to find the container by a predefined name.

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docker container run -it --name dht python:3.7-alpine /bin/ash

The shell should now look like below. We’re now running as a root user in the container.

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/ #

Install Build Dependencies

Build Tools

To build all dependencies we need to be able to build the python lib RPi.GPIO.

Run

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apk update && apk upgrade
apk add g++ python3-dev

to install the build tools.

RPi.GPIO

It cost me some headache and searching around how I get this to build, as I always got errors that gcc exited with status code 1. Luckily I stumbled upon a post in https://archlinuxarm.org/forum/viewtopic.php?p=64598#p64598 by user peiyangxie who gave a one liner how to do this. So let’s do it, run

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CFLAGS="-fcommon" pip3 install RPi.GPIO

in the container. This will install RPi.GPIO.

adafruit-circuitpython-dht

Last but not least we need to install the library adafruit-circuitpython-dht. Do this by running

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pip3 install adafruit-circuitpython-dht

Clean up

As we do not need g++ and python3-dev anymore, we should remove them to shrink container size.

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apk del g++ python3-dev

Then exit the container and return to the host shell.

Build Image from Container

It remains to build the image from the container. This is easily done by

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docker container commit dht dht-image

and then remove the container

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docker container rm dht

Via Dockerfile

To build dht-image automatically, just create a file called Dockerfile and put below input in it.

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FROM python:3.7-alpine

RUN apk update && apk upgrade
RUN apk add g++ python3-dev
# See https://archlinuxarm.org/forum/viewtopic.php?p=64598#p64598
RUN CFLAGS="-fcommon" pip3 install RPi.GPIO
RUN pip3 install adafruit-circuitpython-dht
RUN apk del g++ python3-dev
ENTRYPOINT ["/bin/ash"]

Navigate to the place the Dockerfile is located and run

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docker image build -t dht-image .

to build the image.

Read DHT with dht-image

If you completed the aforementioned section you should have an image called dht-image. Let’s verify this.

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docker images

It should print out a list of images, which contains dht-image (most likely at the top).

In the previous post I shared a script dht.py which we will first download into the container and then change the pin/sensor according to our needs. Finally we run the script and should get results from our sensor.

Note

Docker is not able to access the GPIO pins unless we give it acces. There is a really good, short and comprehensive way about the best options we have on stackoverflow.com/a/48234752. We will use the second option and add /dev/gpiomem as a device to our container, as it is the easiest and most sensible option for our needs.

Start Container

We start the container with /dev/gpiomem as a device and run /bin/ash in it.

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docker container run --device /dev/gpiomem -it dht-image /bin/ash

Download dht script

Download the dht.py script via

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wget https://misterderpie.com/assets/scripts/2021-03-21-raspberry-four-dht/dht.py

Adjust dht script to wiring

The line we need to change is line 8. If you are familiar with Vi you could easily do that, but in case you are not this is also achievable with a single sed command (Source: stackoverflow.com). Run the following command with

• {X} - 11 or 22 (DHT version)
• {Y} - Your GPIO Pin

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sed -i '8s/.*/dhtDevice = adafruit_dht.DHT{X}(board.D{Y})/' dht.py

so for instance, I have a DHT11 and it is connected to Pin 4, so I would run

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sed -i '8s/.*/dhtDevice = adafruit_dht.DHT11(board.D4)/' dht.py

Run dht script

This is the easiest step. Just execute it via

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python3 dht.py

and you should start seeing output.

I do not know why, but this is failing a lot more often than running directly on the Pi. So you most likely see something like this

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/ # python3 dht.py 
Checksum did not validate. Try again.
Checksum did not validate. Try again.
Temp: 69.8 F / 21.0 C    Humidity: 20% 
Checksum did not validate. Try again.
A full buffer was not returned. Try again.
Temp: 69.8 F / 21.0 C    Humidity: 20% 
Checksum did not validate. Try again.
Checksum did not validate. Try again.

Anyway, here we are, having a working docker image in place that returns (most of the times) the results from the sensor.